This invention relates to an epoxy resin composition for cast molding, more particularly to an epoxy resin composition which is excellent in crack resistance, strength and low shrinkage of the cured product, and also good in fluidity.
Attention has been directed recently to epoxy resin compositions for good strength in cured products with little shrinkage during casting, and such compositions have widely been used as castings or moldings. For making casting operation easier, epoxy resins with low viscosity are demanded.
It has been well known in the art that low shrinkage, crack resistance and strength of the cured products can be improved by formulation of fibrous fillers in molding materials or casting materials. Generally speaking, these characteristics can be improved in proportion to the amount of the fibers formulated, but the viscosity of the composition will be increased as a consequence. In particular, fluidity of a casting material will extremely be lowered until it cannot be poured into a mold. For this reason, the amount of the fibers is extremely limited, and the effect of addition cannot entirely be expected.
Accordingly, as the result of studies and developments progressed about the resin having at the same time the strength and low viscosity of the materials in the field of plastics, the following proposal has been made.
In the field of thermoplastic resins, for making better the supplying characteristic during molding, a resin in which fibers with further shorter fiber lengths than chopped strand conventionally used are formulated has been known. However, the feeding characteristic indicates the feeding characteristic from a hopper during injection molding and it is entirely different from the concept of fluidity of a cast molding material. Also, Japanese Provisional Patent Application No. 168619/1983 discloses that "cut glass fibers have the function of improving flowing characteristic of rubbery polymer with high viscosity during heat curing to give good results to sealability and workability, and also prevents generation of bubbles at the adhereing interface, thereby functioning effectively for disapparance of the defects for causing peel-off of the coated film".
However, the supplying characteristic and fluidity mean the flowing characteristic of the material when it flows under pressure during compression molding, injection molding, RIM molding, etc., but not the flowing characteristic of the material which flows naturally under gravity in cast molding. Further, according to the experiments by the present inventors, it has been found that, even when an epoxy resin is formulated with cut glass fibers alone, the viscosity of the composition will be markedly elevated in proportion to the amount of said fibers formulated. Thus, the improved effect of the fluidity described in the above Patent Publication does not appear to be due to the cut glass fibers alone, but it seems to be exhibited by the combination of the glass fibers with the curing agent. However, the improved effect of fluidity by such a combination is very weak and the cut glass fibers, although having very short fiber lengths, are still fibrous, causing a considerable increase in viscosity even when added in a small amount, and therefore the above composition cannot be expected to be useful as a conventional cast molding material.
Accordingly, in this field of the art, it has been earnestly desired to have an epoxy resin composition for cast molding containing a large amount of fibers and also having a low viscosity, while attempting to improve workability during cast molding operation.